In order to comply with the more stringent exhaust gas legislation, nitrogen oxides contained in the exhaust gas must be reduced in internal combustion engines, in particular in self-igniting internal combustion engines. To carry out the necessary exhaust gas aftertreatment, a method based on selective catalytic reduction (SCR) has become very widely accepted. In the SCR method, the nitrogen oxides are reduced into nitrogen and water with the aid of an operating substance and/or additive, in particular a reduction agent. An aqueous urea solution, which is available, for example, under the trade name AdBlue, is often used as the operating substance and/or additive. Thermal decomposition of the urea produces the actual reduction agent, gaseous ammonia, NH3, in the exhaust gas system. The operating substance and/or additive is/are stored in a tank and injected into the exhaust gas system via a delivery or metering module. In this connection, it is always necessary for an adequate amount of reduction agent to be injected by the metering module. To determine the correct amount of the required operating substance and/or additive, the control unit must know the exact properties of the operating substance and/or additive. If the urea content of the frequently used aqueous urea solution is too low, for example, insufficient ammonia is formed in the exhaust gas system and the nitrogen oxides cannot be completely reduced. This has the consequence that the statutory exhaust gas limiting values are exceeded. Causes of an incorrect urea concentration may be, for example, filling an incorrect reduction agent into the tank or deliberately filling the tank with distilled water instead of the operating substance and/or additive due to its significantly lower price. Since the exhaust gas aftertreatment system must ensure compliance with the legal limiting values of nitrogen oxide emissions at all times, it is necessary that the exhaust gas aftertreatment system detects improperly filled operating substances and/or additives, and prevents their use.
German Application No. DE 10 2007 059 853 describes a tank for storing an operating substance or additive, which is equipped with means for measuring the fill level and quality of the liquid in the tank. The measurement is carried out with the aid of an ultrasonic transducer which transmits ultrasonic signals and receives the signals reflected back. From the received ultrasound echoes, it is possible to calculate, among other things, the density of the liquid and the fill level with the aid of the knowledge of the temperature. By comparing the density with stored values, it is possible to infer the nature or quality of the liquid in the tank.
Another approach for determining the quality of the operating substance and/or additive in the tank is to measure the efficiency of the SCR catalyst in reducing nitrogen oxides. The disadvantage of this method is that the discovery of a poor quality of the reduction agent is delayed, i.e., only after it is no longer possible to meet the exhaust gas limiting values, as well as the generally low accuracy of this measurement. All known methods from the related art for measuring the quality of an operating substance and/or additive in a tank exhibit relatively high complexity and expense.